Potentiometers



Dec. 26, 1967 DION 7 3,360,758

POTENTIOMETERS Filed July 28. 1965 Geo/ye; Hen/ f fl/on INVENTORJ ATTORNEY United States Patent Qfiice 3,360,758 POTENTIOMETERS Georges Henri Dion, Garches, Seiue-ebOise, France, as-

signor to Societe dlnstrumentation Schlumherger, Paris, France, a corporation of France Filed July 28, 1965, Ser. No. 475,5tl8 Claims priority, application France, July 30, 1964, 983,549 2 Claims. (Cl. 338-474) ABSTRACT OF THE DISCLOSURE A potentiometer is disclosed which includes a casing having an annular groove formed in the interior casing Wall. The groove opens into the casing to form a pair of spaced-apart parallel and substantially right-angled edges which seat an open ring mandrel of circular crosssection formed of a flexible material and mounting a helically wound resistance wire thereon. The spacing between the edges is larger than the radius and smaller than the diameter of the mandrel cross-section, the mandrel being under compression when forced into said groove and being seated against movement solely through contact with the right-angled edges. Electrical contact is made to a lateral portion of the resistance wire by a rotatable wiper arm having a flexible section in contact with the resistance wire and cantilevered from a more rigid arm.

This invention relates to potentiometers and, more particularly, to improved precision potentiometers capable of withstanding relatively high vibrations and thermal cycles.

A known method of producing a precision potentiometer is to make a toroid from a rectilinear mandrel of circular cross-section covered with an electrical insulating material and on which is wound a small-diametered wire of desired resistance. The mandrel, is then spirally wound to form a helix, one side of each turn of the helix then being out along an axis parallel to the helical axis so as to form a plurality of open-ended ring-like m-andrels.

In this manner many twisted open-ended rings are obtained which are suitable for use as open wound toroids, which must then be secured to the casing of the potentiom eter. Various prior art securing devices have been used, but they are generally complicated and cumbersome. For instance, one conventional method of fixing the toroid in the casing of the potentiometer is to make a V-shaped circular groove in the bottom of the casing and to cement the toroid therein. This method presents known disadvantages, especially with respect to the exact centering of the toroid in relation to the axis of the potentiometer and to the local stresses to which the toroid becomes exposed as a consequence of the non-uniform hardening or of the thermal expansion of the cement.

It is, therefore, a primary object of this invention to overcome these and other disadvantages of the prior art.

It is another object of this invention to provide new and improved potentiometers capable of withstanding acceleration and vibration forces of relatively high amplitude without increasing the potentiometers coefficients of static and dynamic friction.

A further object of this invention is to provide new and improved potentiometers which can continually operate for longer periods than was possible with prior art Potentiometers.

The above and other objects are accomplished in accordance with this invention by providing a potentiometer having a casing in which is enclosed a resistance member consisting of an open toroid of circular crosssection on which is uniformly wound a small-diametered 3,360,758 Patented Dec. 26, 1967 resistance wire. The potentiometer further includes a contact arm and a return arm, the contact arm being at tached to a rotating spindle and designed to slide on the lateral part of the resistance member, while the return arm is connected to an output terminal and is designed to slide on the base of the contact arm.

In accordance with a further feature of the invention, the wound toroid is embedded and maintained by its own elasticity in a uniformly, U-shaped groove which is cut on the inner wall of an insulating support member fixedly secured to the inside of the potentiometers casing.

According to yet another feature of this invention, the width of the groove cut in the support member has a dimension which is greater than the radius but smaller than the diameter of the toroid.

According to another feature of this invention, the depth of the groove in the support member is greater than the segment which sustains the circular arc of the toroid lying between the edges of the groove.

According to another feature of this invention, the insulating support member is made of plastic material and is forcibly inserted into the potentiometers casing.

As a direct consequence of the above-mentioned features, the wound toroid is firmly held in position even under the most severe operating conditions, in particular under acceleration and vibration.

According to another characteristic of this invention, the output terminals of the potentiometer being introduced under pressure into the openings produced in the wall of the potentiometer casing, attachment of these terminals is effected by means of a retaining ring, located in a circular channel, shaped in the casing after the introduction of the terminals, the depth of the channel being governed by the need to produce a flat end in the terminals.

According to a characteristic complementary to the preceding one, the retaining ring of the potentiometer terminals is formed by a rim on a support member. As a consequence, the output terminals of the potentiometer can neither rotate nor be withdrawn.

According to another feature of the invention, the end portion of the contact arm drops inside the toroid, contact with the toroid being effected by means of a precious alloy wire secured to the end of the contact arm. The free end of the contact arm is flattened and cold-hardened to conform to the radial surface of the potentiometer. As a consequence, the life of the potentiometer is considerably increased.

In accordance with a further characteristic of this invention, the contact and return arms each consists of a rigid element to which is secured a flexible strip, terminating in a contact, the length of the strip being less than that of the rigid element. Pursuant to this arrangement, the vibration frequency of each of the arms is considerably increased.

According to another characteristic of the invention, one of the two integral bearings of the potentiometer shaft is mounted in a micrometric screw engaged in a thread drilled in one of the potemtiometers cover plates, the bearings being of the oblique type with a clearance recovery spring, which is supported on a shoulder cut in one end of the shaft.

As a result of this arrangement it is possible, by turning the micrometric screw, to eliminate the axial play of the pair of bearings, mounted on the potentiometer, and thus to permit the movable mechanism of the potentiometer to withstand high frequencies of vibration.

The above and other characteristics of the invention will become apparent from a consideration of the sole figure which represents a radial half-section of the potentiometer.

Referring now to the drawing, the metal casing of the potentiometer has the shape of a cup in the wall 12 to which are fixed output terminals 14, which are mounted in an insulating cylinder 16 which can be produced by milling. After the introduction of the terminal 14, which has been previously mounted in the cylinder 16 or included during the milling process, in the holes produced for this purpose in the wall 12 of the casing 18, a circular throatpiece 18 is inserted into the wall in such a way as to produce a flat surface 20 in the cylinder 16. A support member 22 of plastic material (e.g., Teflon), which may have on the one hand a projecting rim 24 and on the other hand a groove 26, is forcibly inserted into the casing 10, the rim 24 being engaged in the throat-piece 18 and supported on the flat face 20.

The resistance path of the potentiometer, formed by the wound toroid 28, is embedded in the groove 26. The support member 22 may be made either of soft plastic, and in this case a toroid 28 which has been previously corrected for size is forcibly engaged in the groove thanks to the plastic nature of this support, or of hard plastic, even ceramic or oxidized met-a1. In the latter case, the toroid is introduced into the groove and, simultaneously, straightened, which has the effect of increasing the diameter (passing of the helix to the circle) and of arriving at the required position.

The shaft 30 of the potentiometer is mounted on ball bearings 32 and 34. An insulating sleeve 36 is mounted on the shaft and carries a collar 38 which forms the base of the contact arm.

The contact arm consists of a symmetrical, rigid flat piece 40, centered and soldered to the collar 38, and of a flexible strip 42 soldered to the flat piece 40. The radius of the contact arm which consists of the flat piece 40 extended by the strip 42 is less than the inner radius of the toroid. The position of the collar 38 on the shaft 30 is such that the upper face of the rigid element 40 is positioned below the upper plane of the wound toroid.

To the end of the strip 42 is secured a precious alloy wire 44 which constitutes the contact between the contact arm and the toroid 28. The end 46 of the wire 44 is flattened as shown in the drawing and as a result has at least a dimension double to that of the wire diameter.

The return arm consists of a rigid bar 48 riveted to the end 50 of terminal 14, and ends in a flexible strip 52 to which is attached a contact 54 designed to slide over the base of the collar 38. The end 56 of the rigid bar 48 is flattened onto the flat surface 58 produced in the insulating cylinder 16. A plate 60, fixed to the casing 10 by a series of automatically-securing screws such as, 62, completes the potentiometer.

The end 64 of the shaft 30 engages in the oblique ball bearing assembly 34. A shoulder 65 in the end 64 serves as a base for a spring 66 which is in contact with a supporting head 68. The supporting head 68 is in contact with a row of ball bearings 70, designed to move over the oblique bearing assembly 72. The bearing assembly 72 is mounted in a hollow screw 74 with a micrometric thread. After adjustment, the screw 74 is locked in position by means of sealing wax 76.

The fact that the wound toroid 28 is mounted in the groove 26, made in the support member 22, enables axial and radial acceleration of very great amplitude to be applied ot the potentiometer. In fact, the toroid 28, which outside of its position is in the form of an open ring, which has preferably been corrected for size previously, assumes in the support member 22 a closed state and is thus maintained firmly in position in the groove 26 as a result of its own elasticity. A mounting of this sort ensures a uniform distribution of the forces for maintaining the toroid in its support and imparts a particularly high degree of stability to the toroid-support assembly. Furthermore, the fact that the support member is constructed of plastic material results in a better seating of the toroid 28. The width of the groove 26 can obviously be varied, but in regard to the maintenance in position of the wound toroid 28 the optimum value of the width should preferably lie between that of the radius and that of the diameter of the cross-section of the toroid. At the same time, the seating of the toroid 28 in the support member is improved if contact between the toroid 28 and the support member is effected simply on the basis of two circles, i.e., if the depth of the groove 26 is greater than the segment sustaining an are on the cross-section of the toroid falling between the edges of the groove.

It must also be added that the forcible insertion of the semi-flexible support member 22 in the casing 10 of the potentiometer increases still further the effect of the seating of the toroid 28 just as the presence of the plate 6t! contributes to the maintenance of the support member 22 in the casing.

With regard to the attachment of the terminals of the potentiometer, such as 14, in the wall of the casing, it can be seen that the method of attachment is particularly efficient. In fact, the rim 24 of the support member 22 constitutes a retaining ring which holds the output terminals 14 firmly in position. This ring is in fact supported on the flat piece 20, produced in the insulating cylinder 16, thus preventing any movement by the terminal 14 or by the insulating cylinder 16, either by way of rotation or of axial displacement. As a result of this arrangement, the potentiometer can be repeatedly disassembled and reassembled.

On the other hand, since the toroid 28. is supported laterally by the edges of the groove 26, a considerable part of the toroids cross-section is free, thus enabling intermediate potentials to be accepted without difficulty and brought into connection with the output terminals.

The end of the rigid bar 48 of the connecting arm has been pressed flat onto the flat piece '56 at the end of the cylinder 16, thus enabling an effective attachment of the connecting arm to the output terminal 14 to besimply effected. The connecting and contacting arms are composed of the solid pieces 40 and 48 and of the flexible strips 42 and 52, thus enabling the characteristic frequency of vibration of the arms to be considerably increased. As previously mentioned, the length of the strip 42 is preferably selected so that in the absence of the wire 44,

, the strip would be free, that is not in contact with the toroid 28. Since, moreover, the outer part 46 of the contact wire 44 has been flattened in such a way that its hardness and level of wear have been increased and width reduced, the life of the contact 44 is thus considerably increased and, at the saem time, that of the potentiometer as a whole.

The length selected for the flexible strip 42 has an advantage particularly relevant to the length of life of the potentiometer due to the fact that the end of the strip cannot, under any circumstances, make contact with and thus damage the toroid 28. When the wire 44 becomes completely worn out, the strip 42 becomes free and out of contact with the toroid 28, the potentiometer is out of order, but, since the toroid is still serviceable, the potentiometer can easily be reconditioned. It need only be returned to the factory for the contact 44, attached to the strip 42, to be changed. Thus, for a moderate cost, the potentiometer can be reconditioned.

After having increased considerably the characteristic frequency of vibration of the connecting and measuring arms, so that the potentiometer becomes able to stand up to vibrations of particularly high frequency, the longitudinal play of the shaft 30 is substantially eliminated by the clearance recovery bearing 34 above described.

In the case where the shaft 30 passes through the plate 60, the type of oblique bearing for clearance recovery becomes slightly difficult, but its method of operation and efliciency remains substantially the same.

The invention is, of course, not limited to the embodiment above described but may be subject to several modifications without departing from the scope of the appended claims. Thus, for example, in the case of an insulating casing 10 the groove 26 may be cut directly in the wall of the casing. In this instance, an insulating retaining ring in the shape of the rim 24 is used, the flat surface 20 being constructed directly on the terminal 14. On the other hand, it is possible to use a crown type of casing and two lateral plates.

What is claimed is:

1. A potentiometer comprising a casing having a circular inner Wall which includes a circular wall portion composed of an electrical insulating material, said circular portion having an annular groove formed therein, the groove opening into the casing to form a pair of spaced-apart parallel and substantially right-angled edges in said circular wall portion, a resistance member comprising an open ring mandrel of circular cross-section formed of a flexible material and mounting a helically wound resistance wire thereon, the spacing between said edges being larger than the radius and smaller than the diameter of the cross-section of said mandrel, said memher being under compression when forced into said groove and being seated against movement solely by the pair of right-angled edges, a shaft rotatably mounted in said casing, a contact arm formed of a rigid section and a flexible section cantilevered on said rigid section,

both sections extending radially from said shaft, said flexible section diverging from a radial plane which passes through said rigid section and extending to slidably engage a lateral portion of the resistance member, an output terminal and a return arm having two ends, one end of said return arm being fixed to said output terminal and the opposite end thereof making electrical contact with said rigid section of said contact arm.

2. The potentiometer as claimed in claim 1, wherein said contact arm has a length greater than the inner radius and less than the outer radius of said ring mandrel.

References Cited UNITED STATES PATENTS 1,773,427 8/1930 Larsh 308-189 X 2,889,529 6/1959 Vacha 338-202 2,904,767 9/1959 Vacha 338-162 X 3,010,090 11/1961 Vacha 338-202 X 3,139,601 6/1964 Kruse et al. 338l43 3,237,143 2/1966 Baranowski 338315 3,303,453 2/1967 Doering 338162 RICHARD M. WOOD, Primary Examiner.

J. G. SMITH, Assistant Examiner. 

1. A POTENTIOMETER COMPRISING A CASING HAVING A CIRCULAR INNER WALL WHICH INCLUDES A CIRCULAR WALL PORTION COMPOSED OF AN ELECTRICAL INSULATING MATERIAL, SAID CIRCULAR PORTION HAVING AN ANNULAR GROOVE FORMED THEREIN, THE GROOVE OPENING INTO THE CASING TO FORM A PAIR OF SPACED-APART PARALLEL AND SUBSTANTIALLY RIGHT-ANGLED EDGES IN SAID CIRCULAR WALL PORTION, A RESISTANCE MEMBER COMPRISING AN OPEN RING MANDREL OF CIRCULAR CROSS-SECTION FORMED OF A FLEXIBLE MATERIAL AND MOUNTING A HELICALLY WOUND RESISTANCE WIRE THEREON, THE SPACING BETWEEN SAID EDGES BEING LARGER THAN THE RADIUS AND SMALLER THAN THE DIAMETER OF THE CROSS-SECTION OF SAID MANDREL, SAID MEMBER BEING UNDER COMPRESSION WHEN FORCED INTO SAID GROOVE AND BEING SEATED AGAINST MOVEMENT SOLELY BY THE PAIR OF RIGHT-ANGLED EDGES, A SHAFT ROTATABLY MOUNTED IN SAID CASING, A CONTACT ARM FORMED OF A RIGID SECTION AND A FLEXIBLE SECTION CANTILEVERED ON SAID RIGID SECTION, BOTH SECTIONS EXTENDING RADIALLY FROM SAID SHAFT, SAID FLEXIBLE SECTION DIVERGING FROM A RADIAL PLANE WHICH PASSES THROUGH SAID RIGID SECTION AND EXTENDING TO SLIDABLY ENGAGE A LATERAL PORTION OF THE RESISTANCE MEMBER, AN OUTPUT TERMINAL AND A RETURN ARM HAVING TWO ENDS, ONE END OF SAID RETURN ARM BEING FIXED TO SAID OUTPUT TERMINAL AND THE OPPOSITE END THEREOF MAKING ELECTRICAL CONTACT WITH SAID RIGID SECTION OF SAID CONTACT ARM. 